In macro shooting AF is not that important and good quality manual lenses are IMO the best value for a low budget option. The OM 50mm f/3.5 lens is perhaps the best, and the most plentiful on the used market. You can pick up a nice one for under $100 to $140, or less if you take your time, but take care to avoid anything with fungus.

You will need an OM to 4/3 adapter but they can be found for around $10.

And for a few more dollars you can pick up generic OM extension tubes for higher magnification.

These shots were taken with the OM 50/3.5 A lens that cost me just $96